Does anyone know if or when there will be a spanish release of Gospel Library for windows 8.1?

The answer for your question is simple, "Yes, and soon." It's currently being worked on for the rewritten version of the LDS Gospel Library app for Windows 8.1. Unfortunately, we have no ETA for the new version of the app.

The "Gospel Library for Windows 8 New Features" page for the LDS Gospel Library for Windows 8.1 project lists the following for upcoming features:

• Things on the known issues list • New rendering engine that supports new content being published by the Church • Notes and annotations syncing • Multiple language support • Links go to verse and paragraph rather than beginning of document • Bookmarks • Adjust font size • Provide night display mode • Allow copying text from the app to aid those developing lessons

Further, the FAQ states the following:

Q: Currently Gospel Library for Windows is only available in English. Will other languages be available in the future?

A: Yes. The plan is to introduce new languages with the new rendering engine in the next release, or shortly thereafter. We believe that content for the platform will be available initially in at least 8-10 additional languages. The user interface for the application may need translation work. If you would like to help with translation work for the U.I. of the application, please contact the Gospel Library for Windows team at GospelLibrary-Win@ldschurch.org. Your help would be appreciated.

The Public Beta was posted in the Windows Store last Friday. It has content in 106 languages. The UI, however, is only available in 5 languages - English, Spanish, Portuguese, French, and Italian. More UI languages will be coming, hopefully, soon.